Practice. Also, concentrate on active listening. When I focus on someone while they are talking it’s their face in general, both eyes and I would say mouth is even more important, because if someone pronounces something differently than I do, or speaks too softly, the way to “hear” better is watching their mouth. It’s part of why talking on the phone can be more difficult than in person. When I meet someone, and shake hands, I look right into their eyes. When someone during a conversation starts to go off on an important piece of information I shift my body if I’m not already squarely looking towards them, and I meet their eyes—they have my attention, and they know it. Then quickly I can relax a little, it’s not that an intense stare into the eyes has to be ongoing for a long extended period. The gaze comes and goes as you take in what the person is saying, and as you think and respond.